The illumination equipment according to the invention is intended to produce light which resembles natural light as closely as possible, for instance daylight as provided by the sun. The specification proper will be preceded by an explanation of some fundamental terms relating to the colors of light and color rendering so as to illustrate the problems involved and the concept of the invention.
The term "light" is used to designate electromagnetic radiation capable of including the light stimulus in the eye, in other words being visible. As is well known, the visible radiation (light) lies in the wavelength range from about 380 nm to 780 nm.
So-called spectral distributions are indicated to characterize the distribution of the radiant energy of light sources in relation to the wavelength. The corresponding diagrams, as a rule, present the wavelength on the abscissa, while the spectral radiation flux is plotted on the ordinate.
The radiation flux is defined as the power transmitted by radiation. Its unit is the watt (W). The spectral radiation flux is measured in watt per meter (W/m).
The light stimulus caused in the eye by electromagnetic radiation depends not only on the radiation flux but quite essentially also on the spectral composition of the radiation.
In the illumination technique, therefore, a so-called "luminous flux" is defined; it results from an evaluation of the radiation flux by resorting to the spectral luminous efficiency. Further details on this topic may be found in textbooks on light technique, e.g. the "Handbuch der Beleuchtung", Girardet-Verlag, Essen and in German industrial standards DIN 5033 and 6169.
In the light technique, moreover, a so-called color temperature is defined. A light source is compared with the so-called black body radiator (also referred to as Planck's radiator). As is well-known, a Planck's radiator is a temperature radiator which is at thermal equilibrium. The spectral properties of many light sources (such as fluorescent tubes) differ widely from the behavior of a black body radiator. A so-called correlated color temperature is associated with such types of light; it is given by the difference in the type of color sensed, between the type of color of a particular light source and the corresponding point on the Planckian curve, reaching a minimum value.
In the case of incandescent lamps the color locus lies exactly on the curve of Planck's radiator (because a glow lamp is a black body radiator). For this reason, a precise color temperature can be stated for incandescent lamps.
The above mentioned "correlated color temperature" can be calculated by methods such as developed, for instance, by A. R. Robertson ("Computation of Correlated Color Temperature and Distribution Temperature", A. R. Robertson, Journal of the Optical Society of America, November 1968).
The natural daylight provided by the sun changes its spectral composition constantly in the course of the day and, moreover, depends on the weather, clouds, etc. The CIE has defined daylight by formulae by which the spectrum of daylight of any desired color temperature between 4000 K and 25000 K can be determined by calculation.
In the German industrial standards (DIN) the color rendering of light sources is defined as the relation between the original color of an object and its reproduction color, either exclusively under different illumination or in addition after having passed a transmission process. Color rendering properties of light sources are defined by the general color reproduction index "Ra" (see DIN 6169, part 2). Planck's radiator is used as the type of reference light in the process of characterizing the general color rendering index Ra if the light source to be designated has a color temperature of less than 5000 K. If the color temperature is equal to or higher than 5000 K the daylight as defined by the CIE is used for designation.
Therefore, if light is to resemble natural daylight as much as possible, it must be adapted specifically in respect of many parameters because the common artificial light sources differ more or less widely in their above mentioned parameters from the values of natural daylight.
Daylight does not correspond to the light of the black body radiator. The similarity of an artificial light source with daylight in general can be found out by comparing three parameters, namely the color temperature, the general color reproduction index Ra, and the spectral radiation distribution. Artificial light which has a color temperature in correspondence with daylight of more than 5000 K, a very good color rendering index Ra of more than 85, and a spectrum which is similar to that of daylight, will have much resemblance to daylight on the whole.
It is desirable in many fields to produce artificial light which has characteristics as similar as possible to those of daylight. This applies, for example, to fashion show rooms, professional make-up studios, exhibition halls, etc.
It is known that xenon lamps can be used to provide light which largely corresponds to daylight. However, such lamps are used for special purposes only, e.g. with movie theater projectors, because they involve a high risk of explosion and their operating conditions are expensive.
In the journal DIE FARBE 19 (1970), no.1/6, pages 43.gtoreq.76 Gunter Wyszecki describes a combination of an ultraviolet fluorescent lamp and a tungsten halogen lamp. It comprises filtering for the tungsten lamp.